Interesting Photos.
From looking at the “re-structure” of the design, Having experience with several working MP/StG 44 guns for Live fire and Movie Blank fire work,the Photos show what is probably a tool-room built Prototype, using New Tooling, and Greatly Modifying the original design, the major change being the Re-positioning of the Recoil spring Inside the Bolt carrier, instead of being Located inside the Butt tube, with a solid Bolt carrier of the MP44.
Also the trigger mech. is reconfigured, with a rotating Change Lever, rather than a sliding Cross Button, which means a substantial change of the Trigger and discinnector mechanism.

The Barrel has been shortened, I can’t say if the Gas Piston stroke has been lengthened or is original, and many parts details have been changed.

I can’t make out the detail of the Makers marks but the serial/date stamps are clear, if “Hand done”; from a close examination of the Photos, the only original (Highly modified) part is the Upper receiver group; and notable is the addition of a “hold open” slot (Like the MP40); this of course necessitated a rotating cocking handle which is on a ring on the Bolt carrier Piston shank ( on the MP44 it is riveted through the body of the bolt carrier). All the other major parts are new made.

I would say NOT an “amateur Put-together” after WW II; Maybe German,( MauserWerke); French (Manurhin-Vorgrimmler; Spanish ( CETME-Vorgrimmler); or maybe Argentine or Czech Post-war…they were the only four countries to experiment with the Sturmgewehr design after WW II…the French by “capturing” MauserWerke and Vorgrimmler and his StG45, CETME when Vorgrimmler left France (“Escaped”) to Spain; the Czechs by assembling a lot from Parts, and the Argentine Army making them from scratch…probably from German Blueprints etc, but they made quite a few for trials of ammunition and field use (they made the ammo as well). The project was overtaken by the adoption of the FN-FAL in 7,62 Nato, which had been originally developed by FN in 7,9×33 PP43 cartridge. Remaining examples of Argentine Sturmgewehrs are in the Museo de la Nacion, the Premier Military arms Museum in BA. Czech use and built Stg44 examples are in both the Czech Army Museum and the National History Museum in Prague.

All Museum examples so far seen are standard, fixed stock versions.

Several other “StG45″ prototype guns have shown up in the USA in the last 20 years, so some “souveniring” did happen in 1945, not only by French troops, but mostly by US Ordnance Research teams.

The Pistol grip looks very much like that of the “StG45/Gerat 06″ developed by MauserWerke ( with roller lugs aka CETME and H&K designs of mid to later 50s);So it could be a MAUSERWERKE “WORK IN PROGRESS” design of improvements on the Haenel MP44 design.

But with a lot of German Designs of auto weaponry, it was “Too little, too Late”. And thinking that Germany had the Cartridge already nailed down by the late 30s, and a production/test version by 1941-42; They should have already had an “Stg” by the beginning of Barbarossa ( June 41); but as in most wars, the “what Ifs” are a recipe for would be victories which were instead, disasters.

Wars like ww2 are not winned by rifle and handheld firearms solely (and especially modern wars),
but on the other end, japanese firearms is a good example of NOT to do during the war.

As for the rifle, it’s possible that somebody modded it very professionally, trying to falsely sell it after the war as quasi stg44 prototype.
Ian had an article on “stg45 prototype” that is circulating years and years on the internet, and looking for a naive buyer, but apparently it is something from Switzerland.

I believe this may have been one of the guns put together in Spain perhaps in France but suspect Spain that the German engineers put together after the war. The Luftwaffe had already committed to the FG42 by 1943 and this weapon is not on the German priority development list of 1943/44. If it was not on the list no resources could be committed to development. That coupled with the approval of the MP45 (H) in 1945 would have stopped any development on such a weapon. I do know that there were some variations of folding stock MP44’s made in Spain. The ones developed in France were on newly designed weapons including the one for the 7.62×33 (M1 Carbine) round. All that said and done it is an interesting weapon. Harry

The pistol grip looks the same as that on the StG45. There is at least one other kurz short barreled weapon out there made by the Germans. It has sold twice in the last thirty years. It looks like a kurz MP5 predecessor. Who know who made it? Are there any stamps on it?

Anon, you have hit the nail on the head! Reading thru the comments I too was shocked it took this long for it to be identified as a MKb-42(H). Its open bolt safety slot is a dead give away, as they were open-bolt weapons in both semi and full auto, along with the full length gas tube. These are the rarer of the “MP-43/STG-44″ family’s lineage of guns, even though a significant number were ordered (but with delays, and slow early production, not too many were made before production was switched over to the improved MP-43/STG-44 closed bolt configuration). So, I personally think this may be a legitimate German prototype, and not a Bubba’d custom job, as it is one of the rarest and earliest (and less refined) German Assault rifles. Same logic stands that foreign experimenters would have ussd the more refined and common STG-44, or one of the more advanced STG-45 prototypes to experiment with in this way. And just a final, and all be it very improbable theory: perhaps one of the MKb-42 delivered guns was captured/lifted by a partisan who saw fit to mate an MP40 stock to it, to aid its concealabity (but the extra short barrel, gas/site block, and handguard appear original factory built design variations, obviously intentionally built for this specific and unusually short front ended piece, and too professionally well done for a WW2 partisan HacK-Job, and most any other amateur). Fun theory, near nil chance of it though. So, with all that can be seen, and the info ascertained, I would bet the farm as to it being a legit German Prototype MKb-42 folder, either a factory one-off experiment, or part of a small test batch of folder variants of one the rarest and earliest of German Assault Rifles, and of all Assault Rifles. Nice Find! And thanks for sharing this remarkable piece of history and machinery!!!